1. Field of the Invention
This invention relates to the preparation of polydiorganosiloxanes. More particularly, this invention relates to a method for preparing diorganovinylsiloxy terminated polydiorganosiloxanes containing fluorocarbon radicals bonded to silicon.
2. Description of the Prior Art
One method for preparing polydiorganosiloxanes is by the polymerization of cyclic diorganosiloxanes represented by the general formula (R.sub.2 SiO).sub.n where R represents a monovalent unsubstituted or substituted hydrocarbon radical and n typically represents 3 or 4, depending upon the identity of the R radical. Numerous acidic and basic materials have been used to catalyze this polymerization.
The polymerization of diorganocyclosiloxanes in the presence of hydroxides of alkali metals such as potassium and sodium is disclosed in U.S. Pat. No. 2,489,138, which issued to Hyde and Daudt on Nov. 22, 1949 and U.S. Pat. No. 2,490,357, which issued to Hyde on Apr. 7, 1953. Alkali metal catalysts having a pH greater than 12 are disclosed in U.S. Pat. No. 2,634,284, which issued to Hyde on Dec. 6, 1949; cesium hydroxide in U.S. Pat. No. 2,737,506, which issued to Hurd et al. on Mar. 6, 1956; and the polymerization of cyclosiloxanes under superatmospheric pressure in the presence of basic catalysts, including alkali metal hydroxides and silanolates, in U.S. Pat. No. 2,759,007, which issued to Dunham et al. on Aug. 14, 1956. Using a mixture of a polyether and an alkali metal oxide, hydroxide, alkoxide, acyloxide or silanolate to catalyze the diorganocyclosiloxane polymerization is disclosed in Japanese Patent Publication No. 98,798/77.
Polymerization of diorganocyclosiloxanes in the presence of alkali metal, ammonium or phosphonium salts of mercaptans is taught in U.S. Pat. No. 3,243,410, which issued to McVannel on Mar. 29, 1966.
The production of hydroxyl terminated polydiorganosiloxanes from polydiorganosiloxanes in the form of liquids or gums or from cyclic diorganosiloxanes by heating any of these materials in a pressurized reactor in the presence of ammonia as a catalyst is taught in U.S. Pat. No. 3,046,293, which issued to Pike on July 24, 1964.
Among the acid catalysts that have been disclosed for diorganocyclosiloxane polymerization are sulfuric acid, phosphoric acid, phosphorus pentoxide, antimony pentachloride, boric acid, and the Lewis type acids such as boron trifluoride and aluminum trichloride. The use of these catalysts in combination with superatmospheric pressure is taught in U.S. Pat. No. 2,759,008, which issued to Dunham et al. on Aug. 14, 1956.
Acid activated carbon black as a catalyst for diorganocyclosiloxane polymerization is taught in U.S. Pat. No. 3,853,933, which issued to Siciliano on Dec. 10, 1974; using mineral supports coated with polymers having sulfonic or phosphoric acid groups as diorganocyclosiloxane polymerization catalysts is disclosed in U.S. Pat. No. 4,426,508, which issued to Dromard and Millet on Jan. 17, 1984.
Polydiorganosiloxanes wherein at least 50 percent of the repeating units contain a fluorinated hydrocarbon radical of the general formula RfCH.sub.2 CH.sub.2 -- where Rf represents a perfluoroalkyl radical containing from 1 to 10 carbon atoms are characterized by their resistance to swelling and solubilization by liquid hydrocarbons such as gasoline and aviation fuels. Elastomers and resins prepared by curing these fluorine containing polydiorganosiloxanes are particularly desirable if the cured material is used as a coating or encapsulant for electronic devices or other substrates located in the engine compartment of automobiles or other vehicles.
U.S. Pat. No. 3,002,951, which issued to Johannson on Oct. 3, 1961, discloses using alkali metal hydroxides, quaternary ammonium hydroxides or the corresponding silanolates as catalysts for the polymerization of diorganocyclotrisiloxanes wherein the silicon atoms are each bonded to a non-fluorinated aliphatic radical containing fewer than three carbon atoms and a monovalent radical of the formula RfCH.sub.2 CH.sub.2 -- where Rf is defined hereinabove.
U.S. Pat. No. 3,974,120, which issued to Razzano et al. on Aug. 10, 1976 teaches preparing diorganosiloxane copolymers having a viscosity of from 0.1 to 200 Pa.s by polymerizing mixtures containing from 40 to 85 mole percent of an alkyltrifluoropropyl cyclotrisiloxane and from 15 to 60 mole percent of a non-fluorinated diorganocyclotrisiloxane in the presence of (1) an aprotic solvent capable of dissolving both the monomers and the final polymer and (2) an alkyl lithium catalyst. The use of cesium hydroxide, cesium silanolate or a compound of the formula R*.sub.4 B.sup.+- OSiR*.sub.3, where R* represents a hydrocarbyl radical, as a catalyst for the polymerization of mixtures of fluorine-containing and non-fluorinated diorganocyclosiloxanes is taught in U.S. Pat. No. 3,978,104, which issued to Razzano et al. on Aug. 31, 1976. Japanese Patent Publication No. 50056/80 discloses using potassium hydroxide to catalyze the polymerization of mixtures containing 100 parts by weight of fluorine-containing diorganocyclotrisiloxanes and from 70 to 900 parts of non-fluorinated diorganocyclotrisiloxanes.
Vinyl terminated polydiorganosiloxanes can be prepared by polymerizing at least one diorganocyclotrisiloxane using a base catalyst such as potassium silanolate or an acid catalyst such as sulfuric acid and including in the reaction mixture at least an equimolar amount of a hexaorganodisiloxane of the general formula (R".sub.2 ViSi).sub.2 O where Vi represents vinyl and R" represents a monovalent hydrocarbon radical free of ethylenic unsaturation. Alternatively, a hydroxyl terminated polydiorganosiloxane is reacted with a hexaorganodisilazane of the general formula (R".sub.2 ViSi).sub.2 NH.
The present inventor found that the catalyst used to polymerize diorganocyclotrisiloxanes containing a RfCH.sub.2 CH.sub.2 -- radical and a non-fluorinated hydrocarbon radical bonded to each silicon atom has a substantial influence on the thermal stability of cured products, particularly gels, obtained by reacting a vinyl terminated polydiorganosiloxane prepared from one or more of these cyclic disiloxanes with an organohydrogensiloxane in the presence of a platinum catalyst. Polyorganosiloxane gels are easily deformable, elastomeric materials exhibiting a hardness value lower than 25 on the Shore 00 durometer scale. The hardness of some gels is too low for measurement on this scale, and is expressed as the depth to which a penetrometer probe of specified dimensions will penetrate or depress the surface of the gel under a specified loading.
The accompanying examples demonstrate that when conventional non-volatile acidic and basic catalysts of the prior art are used to prepare diorganovinylsiloxy terminated polydiorganosiloxanes wherein at least 50 mole percent of the repeating units contain a fluorinated hydrocarbon radical such as 3,3,3-trifluoropropyl bonded to silicon, gels prepared by reacting these polymers with an organohydrogensiloxane often discolor and harden during relatively short exposures to temperatures of 100.degree. C. and above. Some gels discolor during curing.
An objective of this invention is to provide a method for preparing thermally stable liquid diorganovinylsiloxy terminated polydiorganosiloxanes containing repeating units of the formula R(RfCH.sub.2 CH.sub.2)SiO where Rf represents a perfluorinated hydrocarbon radical containing from 1 to 10 carbon atoms and R represents a monovalent non-fluorinated hydrocarbon radical containing from 1 to 20 carbon atoms.